Differences in the structure and function of mitochondrial components in malignant cells as compared to normal cells will be investigated. By "switching on and off" the malignant phenotype in cells transformed by temperature-sensitive mutants of Rous sarcoma viruses, it will be determined whether such differences can be induced and, furthermore, removed again. The following parameters will be studied: (1) The mechanism of formation of multiple-length mitochondrial DNA (we have found previously a direct correlation of complex mit-DNA accumulation with the manifestation of oncogenic virus transformation; (2) the structure of mit-DNA with respect to altered base sequences, degree of methylation and other factors; (3) the structure and/or activity of mitochondrial proteins and enzymes, in particular those involved in mitochondrial macromolecular synthesis and methylation; (4) the relation of certain viral functions to mitochondrial functions.